THE SMALL INTESTINE 89 



Not all the muscular contractions exhibited by the small 

 intestine have a progressive character. Frequently a loop 

 which contains food will become creased at short intervals 

 by rings of constriction which do not shift their position, 

 but remain stationary for a time. The internal effect is to 

 create a series of small pouches holding separate portions 

 of the chyme. After this condition has persisted for a 

 time the regions originally contracted become relaxed, and 

 new contractions set in at points midway between them. 

 Under the influence of such movements the food is con- 

 stantly shifted about and subdivided, but it is not driven 

 steadily in one direction. This "marking time" on the 

 part of the small intestine is referred to as rhythmic seg- 

 mentation. Inasmuch as it serves to alter the contact 

 relations between the intestinal contents and the lining, 

 it probably favors absorption. Some writers have made 

 much of the effect which these contractions may be as- 

 sumed to have upon the flow of blood and lymph in the 

 walls of the canal. When pressure is applied at brief 

 intervals to tissue containing these fluids the result may be 

 described as a massaging action, hastening the circulation 

 and crowding out some of the lymph. This again must 

 tend to promote the absorption process. 



A longitudinal movement of individual loops is often 

 described. Two neighboring turns of the intestine may be 

 seen to glide the one upon the other, coming to rest after 

 slipping a short distance, and presently reversing the direc- 

 tion of their travel. This form of activity cannot defi- 

 nitely further the progress of the contents, but when it 

 affects segments enclosing liquid material we may suppose 

 that the food and juices are tilted about and brought into 

 relation with the largest possible area of absorbing surface. 



The Secretions Entering the Small Intestine. In 

 Chapter VI it was stated that this part of the canal re- 

 ceives the contributions of the liver and the pancreas, as 

 well as of the microscopic glands in its own mucous mem- 

 brane. The bile and the pancreatic juice, it will be re- 

 membered, enter just below the pylorus. The intestinal 



